In components subjected to high temperatures, high velocity combustion gas and corrosive conditions, such as those hot sections of gas turbines and other heat engines, coatings are frequently used to protect such components from those conditions, improving lifespan and reliability. Environmental barrier coating (EBC) systems are thin ceramic layers, generally applied by plasma spraying and/or physical vapor deposition, including Atmospheric and Low Pressure Plasma Spray, Electron Beam-Physical Vapor Deposition (EB-PVD), or Plasma-Spray Physical Vapor Deposition (PS-PVD), and/or other deposition techniques such as High temperature Vacuum Vapor Deposition, Chemical Vapor Deposition, Magnetron or Cathodic Arc Physical Vapor Deposition, Polymer Derived Coatings and Slurry coatings, that are used to protect monolithic ceramic or ceramic matrix composite (CMC) components, from high temperature, water vapor and/or other corrosive combustion gas attacks in gas turbine or other heat engines, and resistant to thermal cycling and mechanical fatigue operating conditions.
Future development in gas turbine engines will necessitate improvements in conventional environmental barrier coatings designed to protect gas turbine engine hot section Si-based ceramic matrix composite (e.g., SiC/SiC CMC) and monolithic (e.g., Si3N4) ceramic components, in order to meet future engine reduced weight, higher fuel efficiency and lower emission goals. A coating system consisting of a high temperature capable advanced zirconia-based (or hafnia-based) oxide top coat (thermal barrier) and a less temperature capable rare earth silicates and mullite/barium-strontium-aluminosilicate (BSAS)/Si environmental barrier is a state-of-the-art protective T/EBC coating system for the Si-based ceramic applications. The high temperature capability and high stability oxide TBC and lower stability silicate EBC combined system is also the only possible protective coating solution for ceramic components under very high temperature, and/or high gas flow velocity water vapor combustion environments, since the silicate coatings alone cannot effectively protect the component due to the Si species volatility and low temperature capability. In U.S. Pat. No. 7,740,960 to Zhu et al., the entirety of which is incorporated herein by reference, an advanced multilayer graded environmental barrier coating system was disclosed for 3000° F. (1650° C.) environmental barrier coating SiC/SiC turbine ceramic matrix composition applications, including advanced environmental barrier coating top coat, and strain tolerant interlayers, silica activity graded environmental barrier and first generation ceramic and ceramic rare earth silicate based and rare earth aluminosilicate based composite self-healing bond coats.
The developments for advanced turbine environmental barrier coatings will require the advanced environmental barrier coatings capable of achieving 2700°+F. (1482° C.) bond coat temperature and 3000° F. (1650° C.) surface temperatures and with thin coating configurations (typically 5-10 mils, overall coating thickness 127-250 micrometers). Additionally, resistance to impact, erosion and thermo-mechanical fatigue are also becoming critical to ensure the environmental barrier coating-CMC system integrity and durability under realistic engine operating conditions. One major issue for the current environmental barrier coating development is the undesirable low temperature capability silicon or silicon containing bond coat systems, which have a melting point of 1410° C. or below. In addition, bond coats should be dense, possess high strength and low oxygen activity to protect the CMC substrates, thus typical ceramics or ceramic compounds will not have the toughness, strength and critical chemical attributes to meet durability requirements under the thermal cyclic and mechanical fatigue loading at high temperature.
In order to develop the next generation high performance, durable 2700° F. (1482° C.) environmental barrier coating systems, advanced high temperature cable strength, non-silicon based bond coats will be needed to advance the next generation turbine engine technologies.